The present invention relates to a bonding promoter for improving the bondability of a rubber to a steel cord, and a rubber composition and rubber products containing such bonding promoter. In particular, the invention relates to a bonding promoter for reducing a lowering in the bondability of a rubber to a steel cord during the storage or use thereof at a high temperature and a high humidity.
Since steel cords have overall performances as the reinforcement far superior to those of reinforcing cords made of other fibrous materials, they are widely used as the reinforcement for rubber products such as pneumatic tires for automobiles and conveyor belts.
The steel cords to be used in the pneumatic tires and conveyor belts are plated with brass, zinc, bronze or the like in order to promote the bondability thereof to a rubber, and various plating compositions and thicknesses of the plating have been proposed. On the other hand, for the rubbers to be used for coating the steel cords, it has been proposed to incorporate thereinto a cobalt salt of an organic acid, such as cobalt naphthenate, cobalt rosinate, cobalt stearate, cobalt neodecanoate, cobalt octanoate or cobalt propionate as the bonding promoter for enhancing the bondability of the rubbeer to the steel cord. However, these cobalt salts of organic acids have defects that the bondability of the rubber to the steel cord is seriously impaired during the storage or use thereof at a high temperature and a high humidity, while they improve the initial bondability, and that they tend to undergo agglomeration during the storage thereof, forming agglomerates by mutual sticking.
Various bonding promoters were proposed for overcoming these defects. For example, Japanese patent application Kokai publication No. 55-17371 proposed a bonding promoter comprising a C.sub.7 to C.sub.11 carboxylic acid/boron complex which is in a non-adhesive solid form which can be converted into a fluid powder, non-agglomeration particles, tablets or flakes, if necessary. Japanese patent application Kokai publication No. 60-238326 proposed a bonding promoter comprising a reaction product of a cobalt or nickel carboxylate with an alkaline earth metal borate. However, these promoters have defects that their bondability involves a dependency on conditions of vulcanization and that their warm water-resistant bondability is lowered during vulcanization at a high temperature, though they are effective in improving the dondability of rubber to steel cords at a high temperature and a high humidity to some extent.
Japanese patent application Kokai publication No. 60-193701 proposed a borate of a monocarboxylic acid of the following general formula: ##STR2## wherein Y.sub.1, Y.sub.2 and Y.sub.3 may be the same or different from one another and each represents a resin acid radical, a naphthenic acid radical or a monocarboxylic radical having 7 to 11 carbon atoms, with the proviso that at least one of Y.sub.1, Y.sub.2 nd Y.sub.3 represents the resin acid radical or naphthenic acid radical, as a bonding promoter for inhibiting a reduction in the bondability between steel cords used for the production of pneumatic radial tires and a rubber coating them.
However, this compound has defects that the effect of improving the bondability at a high temperature and a high humidity is insufficient, that it has a bondability dependence on conditions of vulcanization and that the warm water-resistant bondability is lowered in vulcanization at a low temperature.